This invention relates to the field of art pertaining to board test fixtures and other mechanical interfaces for electrically interconnecting electronic circuit cards and the like to electrical switching systems.
A board test system consists of numerous electronic sources and detectors which are connected through an electric switch, or scanner, to a plurality of contact points referred to as scanner pins. A board test fixture then provides an interface between these scanner pins and the electronic components located on an electronic circuit card. Since the electronic signals which are used to determine whether the electronic component is operating properly must pass through the board test fixture both on their way to and from the electronic component, the board test fixture must maintain the signal quality of these signals to ensure that the electronic component is not incorrectly diagnosed as operating properly or improperly.
In order to insure maximum signal quality, the length of the signal path between the scanner and the electronic circuit card should be kept as short as possible. This normally dictates a vertical configuration with the board test fixture sitting directly on top of the scanner and the electronic card directly on the fixture. However, any board test fixture must be easy to assemble and maintain in order to be cost effective and many prior art vertical configuration test fixtures have sacrificed assembly and maintainability to obtain short lead length The ability to automate assembly of the fixture is also an important feature.
Various prior art solutions have attempted to address these requirements. A first prior art solution uses a stiff probe pin to conduct the electronic signal directly from a spring loaded scanner pin to the electronic component under test. The probe pin passes through a first plate having a hole for each scanner pin and through a second plate have holes drilled according to the location of the electronic components on the card. This probe pin may be vertical if the component is located directly over the scanner pin, or the probe pin may be at an angle. At an angle, the probe pin may miss the component which it is trying to contact, or may make a high resistance contact, both being undesirable.
A second prior art solution replaces the stiff probe pin with a flexible probe pin. This permits the holes in both the first and second plate to be at right angles to the scanner pins and electronic components assuring a low resistance contact with the electronic component.
Both the first and second prior art solutions are impractical because they require a large number of scanner pins in order for a scanner pin to be located sufficiently close to the component to be able to use a straight or slightly bent pin. More scanner pins mean additional expensive pin electronics. In order to reduce the number of scanner pins, the fixturing method should permit signals from the scanner pins to be routed, or translated, to probe pins located at a different x and y axis positions from the scanner pins with respect to the plane of the probe plate. For incircuit and functional testing, the fixturing method must permit translation between the component location and the scanner pin location.
A third prior art solution permits translation at the expense of long connecting wires. The fixture again has two plates, the first plate drilled according to the locations of the electronic components on the card and the second plate with holes positioned above the scanner pins. A spring loaded probe with a wire wrap post is mounted in the first plate making electrical contact with the electronic components on the card. An interconnect pin with a wire wrap post is mounted in the second plate making electrical contact with the scanner pin. A wire is then wire wrapped between the probe pin and the interconnect pin to complete the electrical connection. This wire is typically quite long to enable the fixture to be opened for easy building and maintenance. Because the wire is long, the electrical performance of the third solution is inferior. A fourth prior art solution is similar to the third prior art solution but uses short wires between the probe pin and the interconnect pin. However, in order to use short wires, the second plate of the fourth solution is divided into interconnect strips. The interconnect strips are arranged in rows below the probe plate. Electrical connections are made one row at a time between the probe pins and the interconnect pins located in these strips. Starting at one end and after each row of connections is made, each strip is sequentially mounted across the bottom of the fixture. The fourth prior art solution, although offering good electrical performance, is difficult to wire, especially if there is a large number of x and y axis translations. This solution is impossible to automate and service is very difficult.
The fifth and final prior art solution is referred to as the "basic matrix". This solution uses a printed circuit board to perform the x and y axis translation. The probes which contact the electronic components are mounted directly to the top of the printed circuit board and the scanner pins contact the bottom of the printed circuit board to complete the connection. This solution is expensive requiring a custom printed circuit board for each fixture and special probes. Furthermore, this solution causes scanner pins located directly under probes to be made unusable, a very undesirable feature.
A need exists for a low cost, easy to build test fixture for which assembly is automatable. The test fixture should minimize the loss of scanner pins and provide for translation in the x and y axis directions between probe and scanner pin.